The need to power portable electronics equipment, communications gear, medical devices and other equipment in remote field service has been on the rise in recent years, increasing the demand for highly efficient, mobile power systems. These applications require power sources that provide both high power and energy density, while also requiring minimal size and weight, low emissions and cost.
To date, batteries have been the principal means for supplying portable sources of power. However, due to their size and weight, as well as the time required for recharging, batteries have proven inconvenient for continuous use applications. Moreover, portable batteries are generally limited to power production in the range of several milliwatts to a few watts and thus cannot address the need for significant levels of mobile, lightweight power production.
Small generators powered by internal combustion engines, whether gasoline- or diesel-fueled have also been used. However, the noise and emission characteristics of such generators have made them wholly unsuitable for a wide range of mobile power systems and unsafe for indoor use. While conventional heat engines powered by high energy density liquid fuels offer advantages with respect to size, thermodynamic scaling and cost considerations have tended to favor their use in larger power plants.
In view of these factors, a void exists with regard to power systems in the size range of approximately 50 to 500 watts. Moreover, in order to take advantage of high energy density liquid fuels, improved fuel preparation and delivery systems capable of low fueling rates are needed. Additionally, such systems must also enable highly efficient combustion with minimal emissions.
A combustion device wherein fuel is atomized by an ultrasonic atomizing device is proposed in U.S. Pat. No. 5,127,822. According to this patent, atomizers have been proposed wherein fuel is supplied to a combustion chamber in fine droplets to accelerate vaporization of the fuel and reduce the combustor residence time required to achieve acceptable combustion efficiency.
U.S. Pat. No. 5,127,822 proposes an arrangement wherein fuel is supplied at 5 cc/min and the fuel is atomized into droplets having a Sauter Mean Diameter (SMD) of 40 μm. Other atomizing techniques are proposed in U.S. Pat. Nos. 6,095,436 and 6,102,687. An ultrasonic atomizer for supplying fuel to an internal combustion engine is proposed in U.S. Pat. No. 4,986,248.
U.S. Pat. No. 4,013,396 proposes a fuel aerosolization apparatus wherein a hydrocarbon fuel (e.g., gasoline, fuel oil, kerosene, etc.) is dispensed into a condensation area with the intention of forming an aerosolized fuel of relatively even sized droplets less than 1 μm in diameter.
A fuel-vaporizing device said to address problems associated with incomplete combustion of fuel aerosols in internal combustion engines is proposed in U.S. Pat. No. 5,472,645. According to U.S. Pat. No. 5,472,645, because aerosol fuel droplets do not ignite and combust completely in internal combustion engines, unburned fuel residues are exhausted from the engine as pollutants such as hydrocarbons (HC), carbon monoxide (CO) and aldehydes with concomitant production of oxides of nitrogen (NOx). The proposal of U.S. Pat. No. 5,472,645 is intended to improve combustion of aerosol fuels by breaking liquid fuel down into an air-fluid stream of vaporized or gas-phase elements containing some unvaporized aerosols containing hydrocarbons of higher molecular weight, the lighter fuel distillates said to quickly evaporate to the gas phase, mix with air and are to be fed to an internal combustion engine while the heavier fuel portions are said to be transformed into a gas-phase vaporized state before they exit a cyclone vortex device and enter the intake manifold of the engine.
U.S. Pat. No. 4,344,404 proposes an apparatus for supplying aerosol fuel droplets mixed with air to an internal combustion engine or burner, the fuel droplets said to have sizes of 0.5 to 1.5 μm. The liquid fuel in aerosol form is intended to be mixed with air in a air-to-fuel ratio of about 18:1 to produce the least CO, HC and NOx emissions from the engine.
Various devices have been proposed for heating fuels into a vaporized fuel that is combusted by a burner. See, for example, U.S. Pat. Nos. 4,193,755; 4,320,180; and 4,784,599.
U.S. Pat. No. 3,716,416 discloses a fuel-metering device intended for use in a fuel cell system. The fuel cell system is intended to be self-regulating, producing power at a predetermined level. The proposed fuel metering system includes a capillary flow control device for throttling the fuel flow in response to the power output of the fuel cell, rather than to provide improved fuel preparation for subsequent combustion. Instead, the fuel is intended to be fed to a fuel reformer for conversion to H2 and then fed to a fuel cell. In a preferred embodiment, the capillary tubes are made of metal and the capillary itself is used as a resistor, which is in electrical contact with the power output of the fuel cell. Because the flow resistance of a vapor is greater than that of a liquid, the flow is throttled as the power output increases. The fuels suggested for use include any fluid that is easily transformed from a liquid to a vapor phase by applying heat and flows freely through a capillary. Vaporization appears to be achieved in the manner that vapor lock occurs in automotive engines.
U.S. Pat. No. 6,276,347 proposes a supercritical or near-supercritical atomizer and method for achieving atomization or vaporization of a liquid. The supercritical atomizer of U.S. Pat. No. 6,276,347 is said to enable the use of heavy fuels to fire small, light weight, low compression ratio, spark-ignition piston engines that typically burn gasoline. The atomizer is intended to create a spray of fine droplets from liquid, or liquid-like fuels, by moving the fuels toward their supercritical temperature and releasing the fuels into a region of lower pressure on the gas stability field in the phase diagram associated with the fuels, causing a fine atomization or vaporization of the fuel. Utility is disclosed for applications such as combustion engines, scientific equipment, chemical processing, waste disposal control, cleaning, etching, insect control, surface modification, humidification and vaporization.
To minimize decomposition, U.S. Pat. Nos. 6,276,347 and 6,390,076 each propose keeping the fuel below the supercritical temperature until passing the distal end of a restrictor for atomization. For certain applications, heating just the tip of the restrictor is desired to minimize the potential for chemical reactions or precipitations. This is said to reduce problems associated with impurities, reactants or materials in the fuel stream which otherwise tend to be driven out of solution, clogging lines and filters. Working at or near supercritical pressure suggests that the fuel supply system operate in the range of 300 to 800 psig. While the use of supercritical pressures and temperatures might reduce clogging of the atomizer, it appears to require the use of a relatively more expensive fuel pump, as well as fuel lines, fittings and the like that are capable of operating at these elevated pressures.
Power conversion arrangements are proposed in U.S. Pat. Nos. 4,638,172; 5,836,150; 5,874,798; 5,932,940; 6,109,222; and 6,198,038. Of these, U.S. Pat. No. 4,638,172 proposes a direct current generator operatively coupled to a small internal combustion engine, the generator said to output between 4 volts (V) and 150 milliamperes (mA) to 110 V and over 250 mA. U.S. Pat. No. 5,836,150 proposes a micro thrust and heat generator that can be used as a thrust source for a micro machined turbo-electric generator. U.S. Pat. No. 5,874,798 proposes a micro-turbine generator device wherein air is fed into the device to generate electricity for use with portable electronic products. U.S. Pat. No. 5,932,940 proposes a micro-gas turbine engine including a combustion chamber used to drive a microgenerator which is intended to output 10 to 30 watts of electrical power for replacement of batteries in portable electronic devices while producing 20 times the power for the same weight and volume (e.g., replacing batteries for portable computers, radios, telephones, power tools, heaters, coolers, military applications, etc.). U.S. Pat. No. 6,109,222 patent proposes a micro heat engine that is intended to generate 10 to 30 watts of electrical power wherein a free piston is reciprocated by a periodic combustion process.